1. Field of the Invention
This invention relates generally to radio telecommunication systems and more particularly to a system and method for implementing a cellular radio transmitter device.
2. Description of the Background Art
Modern cellular radio telephones provide a portable method of remote communication without the constraint of remaining at a fixed location. This remote communication is accomplished by transmitting radio signals between individual radios each containing a transceiver (a combined receiver and transmitter) which allows a radio user to both speak (transmit) and listen (receive). System performance and system cost are two important factors which significantly impact radio designers, manufacturers and users.
FIG. 1 is a schematic diagram of a prior art transceiver 110 for receiving and transmitting radio signals using a conventional heterodyning technique. Transceiver 110 uses the heterodyning technique to down-convert the frequency of received radio signals and to up-convert the frequency of transmitted radio signals. Heterodyning is a conventional mixing technique in which two input signals are combined to produce output signals having frequencies equal to both the sum and the difference of the two input signals.
In transmit mode, up-converter 142 heterodynes (combines) the transmit signals (I and Q) and a fixed oscillator 150 output signal to produce a transmit signal at the assigned transmit frequency. The transmit signal is amplified through driver 144 and power amplifier 148 and then broadcast through antenna 111. In receive mode, the frequency of the received radio signal is down-converted in three stages. First down-converter 116 initially heterodynes the received signal with an oscillator 118 output signal to produce a first intermediate frequency signal. Second down-converter 122 then heterodynes the first intermediate frequency signal with an oscillator 124 output signal to produce a second intermediate frequency signal. Finally, baseband down-converter 130 heterodynes the second intermediate frequency signal with an oscillator 132 output signal to produce conventional baseband signals I and Q which are then processed into conventional audio signals.
FIG. 2 is a schematic diagram of a prior art transmitter device which compensates for non-linearity in the transmitter power amplifier. Some digital cellular telephone systems make use of angle (phase) modulation techniques in their transmitter sections. In the United States, the Code Division Multiple Access (CDMA) system standard calls for the use of Differential Quaternary Phase Shift Keying (DQPSK) modulation. It is well known that the power amplifier sections of radio transmitters intended for use in these systems must operate in a linear manner in order to faithfully reproduce the carrier amplitude variations that are a characteristic of this modulation scheme.
A well-known method for achieving linear operation of power amplifiers involves running the active devices (transistors or vacuum tubes) at high levels of bias current in order to increase their dynamic range. This method is inappropriate in portable equipment, such as cellular radio telephones, because of the adverse effect on battery life. A method is therefore required whereby a power amplifier design, having good DC to RF efficiency (ie., class AB operation), may be made to exhibit linear gain characteristics within the required operating range.
This problem has been addressed previously using a technique commonly known as "Cartesian Feedback," which has been discussed in Polar Loop Transmitter, Petrovic and Gosling, Electronic Letters, 1979, 15, (10), pp. 286-288, and which is hereby incorporated by reference. The particular implementation shown in FIG. 2 uses a "feedback phase correction" method as disclosed in International Patent Application No. WO 94/05078, entitled "Apparatus For Compensation Of Phase Rotation In A Final Amplifier," filed on Aug. 2, 1993, by Ericsson, Bergsten and Nystrom, which is hereby also incorporated by reference.
In FIG. 2, baseband I input signal is fed through amplifier 200 to an I mixer in conventional up-converter/phase modulator 202. A baseband Q input signal is likewise fed through amplifier 204 to a Q mixer in up-converter/phase modulator 202. An oscillator 220 output signal is rotated 90 degrees out-of-phase and heterodyned with the Q signal inside the Q mixer. An in-phase oscillator 220 output signal is likewise heterodyned with the I signal inside the I mixer. The outputs of the I and Q mixers in up-converter/modulator 202 are then combined and up-converted again in mixer 206 before being amplified by power amplifier 210 to obtain a transmit signal. Power amplifier 210 is operated in an energy-efficient mode which disadvantageously creates non-linear distortion in the transmit signal.
The FIG. 2 transmitter therefore samples the distorted transmit signal using sampler 212, and down-converts the sampled signal using mixer 216. I and Q error signals are obtained using down-converter/demodulator 218 which essentially reverses the process of up-converter/modulator 202. The I and Q error signals are then fed back into the respective amplifiers 200 and 204 to constitute a negative feedback loop and compensate for the non-linear characteristics of power amplifier 210. Furthermore, to compensate for the timing differences introduced by the propagation delay of the feedback path, phase detector 221 controls phase shifter 222 to advantageously synchronize the I and Q input signals and the I and Q error signals.
However, the FIG. 2 transmitter implementation described above increases the complexity of the transmitter circuitry to such an extent that the technique becomes impractical in portable cellular radio handsets. Therefore, an improved system and method is needed for implementing a cellular radio transmitter device.